2013-10-30_REVISION - M1977310 (3)

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Whetstone Associates Technical Memorandum appropriate analyses demonstrating adequate protection is provided by the specified channel protection. Cotter response: a. Table 14 contains WinTR -55 model output, with the 10 -11 and 20 -11 depths truncated /removed. In the NRCS WinTR -55 model, the user specifies channel bottom width and side slope ratio, and the model generates a stage - capacity table for flow depths of 0, 0.5, 1, 2, 5, 10, and 20 feet. The 5 -ft depths were shown in the table because portions of the UDS, MDS, LDS -E, and LDS -N exceed 2 ft in depth. The table was not meant to imply that all channel segments were 5 -ft in depth. A footnote has been added to the table in the revised DDP. The constructed channel depths were surveyed at eight profile locations along the Upper Diversion Structure, seven profile locations along the Middle Diversion Structure, four profile locations along the Lower Diversion Structure — East and six profile locations along the Lower Diversion Structure — North. The channel depths, are summarized in the revised DDP. b. A stability analysis was conducted using the Thorne and Zevenbergen (1985) resistance equations to compute the mean velocity and Fronde number, which were then compared to the Maximum Permissible Velocity or the Fronde number established for channel stability. The Urban Storm Drainage Criteria Manual (UDFCD 2001, 2008), recommends that structures be designed with a permissible velocity of 5 ft /sec or Froude number of 0.6 for soil types A and B and of 7 ft /sec or Fronde number of 0.8 for soil types C and D. Engineered channels at the JD -6 Mine are constructed in soil types C and D (rock) and are thus designed to meet the v = 7 ft /sec or Fr = 0.8 criteria in erodible soils. Results indicate that the existing MDS, LDS -E, and LDS -N diversion structures are capable of conveying the 100 -year 24 -hour storm with an average velocity of less than the design criteria of 7 ft /sec. The average calculated velocity for the MDS, LDS -E, and LDS -N profiles was 3.20 ft /sec and the maximum calculated velocity was 5.79 ft /sec. Results indicate that the UDS, as currently constructed, is incapable of conveying the 100 -year 24 -hour storm with an average velocity of less than 7 ft /sec. The UDS channel has been redesigned with shallower side slopes (3:1), a shallower local bed slope, and six check/drop structures to dissipate grade and energy. The allowable bed slope for the D84 grain size of 25 mm is 0.035 ft /ft. This slope will be graded into native material between the drop structures, as shown in Plate 3 of the revised DDP. 11. Page 26, SWRP spillway. A spillway is required for retention ponds. The DRMS acknowledges the Operator is committing to construct the recommended spillway upon approval of this permit amendment. - However, design drawings stamped and signed by a 4148A.131030 7